1. Field of the Invention
The present invention relates to an improved method and apparatus for magnetic resonance imaging and spectroscopic analysis of a wide variety of specimens and is in one embodiment employable with small blood vessels in determining the presence of atherosclerotic plaque and the composition thereof.
2. Description of the Prior Art
The advantageous use of magnetic resonance technology in providing safe, rapid images of a patient has long been known. It has also been known to employ magnetic resonance technology in producing chemical shift spectra to provide information regarding the chemical content of a material.
In a general sense, magnetic resonance imaging involves providing bursts of radio frequency energy on a specimen positioned within a main magnetic field in order to induce responsive emission of magnetic radiation from the hydrogen nuclei or other nuclei. The emitted signal may be detected in such a manner as to provide information as to the intensity of the response and the spatial origin of the nuclei emitting the responsive magnetic resonance signal. In general, imaging may be performed in a slice or plane, in multiple planes, or in a three-dimensional volume with information corresponding to the responsively emitted magnetic radiation being received by a computer which stores the information in the form of numbers corresponding to the intensity of the signal. The pixel value may be established in the computer by employing Fourier Transformation which converts the signal amplitude as a function of time to signal amplitude as a function of frequency. The signals may be stored in the computer and may be delivered with or without enhancement to a video screen display, such as a cathode-ray tube, for example, wherein the image created by the computer output will be presented through black and white presentations varying in intensity, or through color presentations varying in hue and intensity. See, generally, U.S. Pat. No. 4,766,381.
One of the beneficial end uses of the present invention is in connection with atherosclerotic disease which is a major cause of mortality and morbidity in the United States. Localized forms of the disease, such as the deposit of plaque on the walls of blood vessels, can restrict local blood flow and require surgical intervention in some instances. While angiography is an effective means for detecting the luminal narrowing caused by plaque, it does not provide information regarding the nature of the process leading to blood flow reduction. Unfortunately, therapeutic methods, such as intravascular intervention, may experience failure due to the lack of sufficiently precise imaging methods. An imaging system capable of providing detailed, qualitative and quantitative data regarding the status of vascular walls at the time of surgical intervention, could favorably influence the outcome by enabling the selection of the intervention method to be customized to the particular need. It would also serve to provide precise guidance for various forms of localized therapy.
It has been known to use angioplasty and intravascular ultrasound for imaging plaques. See, generally, Spears et al., "In Vivo Coronary Angioscopy," Journal of the American College of Cardiology, Vol. 1, pp. 1311-14 (1983); and Wailer et al., "Intravascular Ultrasound: A Histological Study of Vessel During Life," Circulation, Vol. 85, pp. 2305-10 (1992). Intravascular ultrasound, however, provides several drawbacks, including the insensitivity to soft tissue and the inability to reliably detect thrombus and discriminate thrombus (new or organized) superimposed upon plaque from soft lipid-laden plaques. Also, the presence of artifacts related to transducer angle relative to the vessel wall, and an imaging plane limited to the aperture of the transducer in variable resolution at different depths of view are further problems with this approach.
The feasibility of identification of atherosclerotic lesions by employing magnetic resonance (MR) microimaging in vitro has previously been suggested. See, for example, Pearlman et al., "Nuclear Magnetic Resonance Microscopy of Atheroma in Human Coronary Arteries," Angiology, Vol. 42, pp. 726-33 (1991); Asdente et al., "Evaluation of Atherosclerotic Lesions Using NMR Microimaging," Atherosclerosis, Vol. 80, pp. 243-53 (1990); and Merickel et al., "Identification and 3-d Quantification of Atherosclerosis Using Magnetic Resonance Imaging," Comput. Biol. Med., Vol. 18, pp. 89-102 (1988).
It has also been suggested that MRI can be used for quantification of atherosclerosis. See, generally, Merickel et al., "Noninvasive Quantitative Evaluation of Atherosclerosis Using MRI and Image Analysis," Arteriosclerosis and Thrombosis, Vol. 13, pp. 1180-86 (1993).
Yuan et al., "Techniques for High-Resolution MR Imaging of Atherosclerotic Plaques," J. Magnetic Resonance Imaging, Vol. 4, pp. 43-49 (1994) discloses a fast spin echo MR imaging technique to image atherosclerotic plaques on an isolated vessel that has been removed by carotid endarterectomy. As the signal-to-noise ratio (SNR) decreases with the decrease in imaging time and increase in resolution, special radio frequency (RF) receiver coils were designed. The article suggests that by the use of special MR hardware at 1.5 T using various T1 and T2-weighted pulse sequences, it is possible to discriminate foam cells, fibrous plaque organized thrombus, new thrombus, loose necrosis and calcium.
It has also been suggested that the fat content of atherosclerotic plaque in excised tissue samples can be determined using chemical shift imaging or chemical shift spectroscopy. See, generally, Vinitski et al., "Magnetic Resonance Chemical Shift Imaging and Spectroscopy of Atherosclerotic Plaque," Investigative Radiology, Vol. 26, pp. 703-14 (1991); Maynor et al., "Chemical Shift Imaging of Atherosclerosis at 7.0 Tesla," Investigative Radiology, Vol. 24, pp. 52-60 (1989); and Mohiaddin et al., "Chemical Shift Magnetic Resonance Imaging of Human Atheroma," Br. Heart J., Vol. 62, pp. 81-89 (1989).
The foregoing prior art articles in the aggregate could lead one skilled in the art to conclude that MR, while having potential for fully characterizing vessel wall disease, suffers from low anatomic resolution unless used in vitro on small specimens with high resolution methods.
It is known that in order to obtain the desired high-resolution imaging and spectroscopy of arteriosclerotic plaques, a coil can be placed close to the target blood vessel.
In Kantor et al., "In vivo .sup.31 P Nuclear Magnetic Resonance Measurements in Canine Heart Using a Catheter-Coil," Circulation Research, Vol. 55, pp. 261-66 (Aug. 1984), there is disclosed an effort to improve the SNR in the .sup.31 P spectroscopy of a dog myocardium using an elliptical coil. This coil is rigid, rather bulky, and designed for spectroscopy of the myocardium, but is not ideal for vessels.
Disclosures of efforts to develop catheter coils for imaging vessel walls are contained in Martin et al., "MR Imaging of Blood Vessel with an Intravascular Coil," J. Magn. Reson. Imaging, Vol. 2, pp. 421-29 (1992); and Hurst et al., "Intravascular (Catheter) NMR Receiver Probe: Preliminary Design Analysis and Application to Canine Iliofemoral Imaging," Magn. Reson. Med., Vol. 24, pp. 343-57 (April 1992). These disclosures employ two tiny diameter, back-to-back solenoid coils to produce a good axial profile when the coils are placed along the main magnetic field.
Martin et al., "Intravascular MR Imaging in a Porcine Animal Model," Magn. Reson. Med., Vol. 32, pp. 224-29 (August 1994) discloses use of the system disclosed in the above-cited Martin et al. article for high-resolution images of live animals. See, also, Abstract, McDonald et al., "Performance Comparison of Several Coil Geometries for Use in Catheters," RSNA 79th Scientific Meeting, Radiology, Vol. 189(P), p. 319 (November 1993). A strong disadvantage of these disclosures is that multislice acquisition cannot be carried out because the longitudinal coverage of the sensitive regions is limited to a few millimeters. Furthermore, the coil itself does not have the desired flexibility while maintaining the desired efficiency of data acquisition.
U.S. Pat. No. 5,170,789 discloses a nuclear magnetic resonance (NMR) coil probe, in the form of a loop, that is said to be insertable within a specimen, which has an opening, for purposes of nuclear magnetic resonance spectroscopy (NMRS). The disclosed two component probe, which is in the nature of an endoscope to examine the colon or cervix, has a first portion which is insertable into a body cavity and a second portion which is external to such cavity. The probe has a flexible coil body with an oval or circular shape that may deform during insertion. As a result, the coil may require tuning after insertion. If the coil were made of a very rigid material, insertion problems may occur. Also, a tuning and matching circuit, in the external portion, may limit the depth of insertion.
U.S. Pat. No. 4,932,411 discloses a probe with a transmit/receive coil for insertion in channels which are surgically or otherwise inserted in body organs, such as the brain, liver or kidneys. The coil, which is in the form of a loop, is carried and wound on the distal end of a carrier which is used to insert the coil into the body channel.
U.S. Pat. No. 4,672,972 discloses an NMR probe disposed at the distal end of a catheter or endoscope for obtaining NMR spectra from within a patient. The multi-turn probe has a parametric amplifier and/or a gate-array attached to it and, also, has a coil cooling system.
U.S. Pat. No. 5,413,104 discloses an invasive MRI transducer having a balloon, at least one lumen, and a flexible coil loop for insertion in a body cavity.
It has been known to employ an MR-active invasive device with RF transmitter coils for selective MR angiography of blood vessels. See, generally, U.S. Pat. No. 5,447,156.
It has also been known to employ an intravascular catheter with a Faraday cage to prevent RF energy from acting on fluid, such as blood, and cause the MR signal to be stronger for the fluid exiting the cage. See, generally, U.S. Pat. No. 5,419,325.
MR compatibility characteristics of various catheter and guide wire systems, for use in interventional MR procedures, has been considered. See Dumoulin et al., "Real-time Position Monitoring of Invasive Devices Using Magnetic Resonance," Magnetic Resonance in Medicine, Vol. 29, pp. 411-15 (March 1993); and Abstract, Koechli et al., "Catheters and Guide Wires for Use in an Echo-planar MR Fluoroscopy System," RSNA 79th Scientific Meeting, Radiology, Vol. 189(P), p. 319 (November 1993).
McKinnon et al., "Towards Visible Guidewire Antennas for Interventional MRI," Proc. Soc. Mag. Res., Vol. 1, p. 429 (August 1994) discloses guidewire antennas which are asserted to promise making Guidewire, biopsy needles and other vascular interventional devices visible by MRI. One MRI stub antenna is a length of coaxial cable with 10 cm of the braid removed from the end. One end of the coaxial cable is directly connected to the surface coil input of an MRI scanner and the other end is placed in a water filled phantom. The MR image is a bright line corresponding to spins in the immediate neighborhood of the cable. A preferred MRI stub antenna is an unterminated twisted pair cable having a diameter of 0.2 or 1 mm, and a corresponding image line width of 1 or 3 mm, respectively, which provides a finer image than the coaxial cable stub antenna. A preferred combination is a steerable guidewire containing a twisted pair cable. It is suggested that a surface coil could be used simultaneously with a guidewire antenna by combining, as with phased array coils, the specimen image from the surface coil with the image of the stub antenna using the data acquired from the stub antenna, to localize the in vivo device during interventional MRI.
It has been known to employ an invasive device having an RF coil for transmitting RF signals which are detected by external RF receive coils to track the invasive device. See, generally, U.S. Pat. No. 5,437,277.
It has also been known to employ external RF transmitter/receiver coils. See, generally, U.S. Pat. No. 5,447,156.
U.S. Pat. No. 5,323,778 discloses a probe for insertion in an artery or other body passageway. The probe has an MRI coil, an external MRI RF source and an RF heating apparatus for hyperthermia therapy.
U.S. Pat. No. 5,358,515 discloses a microwave hyperthermia applicator for limited heating of cancerous tissue including upper and lower dipole halves of the same diameter. The upper dipole half is a widened metal extension of the inner conductor of an insulated coaxial cable. The lower dipole half is a metal cylinder connected to the outer sheath of the coaxial cable. A .pi./2 (.lambda./4) transformer, such as the outermost metal cylindrical sheath of a triaxial cable, is separated at its upper end from the lower dipole half which is connected to the coaxial cable outer sheath. The transformer is filled with a dielectric medium and is connected at its lower end to such coaxial cable outer sheath. When the antenna is inserted in a dissipative medium and supplied with microwave energy through the coaxial cable, only that area of the medium immediately around the antenna is heated.
MRI has many desirable properties for the diagnosis and therapy of atherosclerotic disease. For example, it is possible to see lesions directly, even before the plaques calcify. However, the SNR of MR images obtained from conventional surface or body coils is insufficient. This is because the coils placed outside the body pick up noise from a very large region of the body. To achieve satisfactory quality, the signal receiver can be placed as close as possible to the tissue of interest (e.g., blood vessels). A coil placed on the tip of a catheter and inserted into the blood vessels could be a solution; but, the real part of the impedance of a catheter coil is relatively small and, hence, a tuning and matching circuit is preferably located immediately after the coil within the blood vessels. It is believed that prior art designs that do otherwise suffer from a significant SNR loss. On the other hand, it is believed that prior art designs, which have a tuning and matching circuit immediately after the coil in blood vessels, are too thick to be placed into small vessels.
There remains, therefore, a very real and substantial need for an improved apparatus and method for MR imaging and spectroscopic analysis of specimens in a manner which provides efficient data acquisition with maximum SNR while permitting in vivo or in vitro acquisition from small vessels and a wide range of other types of specimens.